This invention relates to a multi-piece golf ball of at least three layer structure.
Because of the distance advantage, solid golf balls have replaced wound golf balls and become predominant in the golf ball art. The solid golf balls having solid cores include two-piece golf balls of two-layer structure, three-piece golf balls of three-layer structure, and multi-piece golf balls of four or more layer structure. Even though solid golf balls were deemed inferior to wound golf balls with respect to the spin performance upon approach shots, such shortcomings of solid golf balls were overcome by recent advances including the development of a soft cover.
Nevertheless, many professional golfers and low-handicap skilled golfers still favor wound golf balls. It is believed that this choice depends not only on the performance on approach shots, but also on restitution or rebound in a high head speed region. The restitution of wound golf balls is less dependent on the head speed. When hit in the high head speed region, wound golf balls experience that the reduction of restitution is small and thus travel good distance, as compared with solid golf balls. This behavior is characteristic of wound golf balls, and it is believed difficult to impart this behavior to solid golf balls.
An object of the invention is to provide a multi-piece golf ball of at least three layer structure whose performance is less dependent on a head speed and which exhibits improved flight performance in high head speed region and the accuracy of strike corresponding to a particular type of club.
The invention is directed to a multi-piece golf ball of at least three layer structure comprising a core, an intermediate layer enclosing the core, and a cover enclosing the intermediate layer. If the firing velocity used in the measurement of a coefficient of restitution (COR) and the head speed (HS) upon hitting with a given club are of the same value, the deformation behaviors of the ball upon impact are substantially identical between the measurement of COR and the club hitting. It has been found that the dependency of ball performance upon head speed is reduced as the difference between a COR value of a ball in a low HS region (typically 25 m/s) and a COR value in a high HS region (typically 50 m/s) is minimized, that is, the difference between COR upon low HS club hitting and COR upon high HS club hitting is minimized. More specifically, it has been found that when the ratio of A/B is set to be 1.10/1 or greater wherein A is a deflection (mm) under an applied load of 100 kg of the core and B is a deflection (mm) under an applied load of 100 kg of a sphere having the core enclosed with the intermediate layer, and the difference between the COR at the low firing velocity and the COR at the high firing velocity is less than 0.100, quite unexpectedly there is obtained a multi-piece golf ball whose performance is less dependent on a head speed and which exhibits improved flight performance in the high head speed region and the accuracy of strike corresponding to a particular number of club.
It has also been found that better results are obtained by specifying additional factors including the total volume of dimples in the cover, the material and Shore D hardness of the cover, the deflection of the core, and the materials of which the core and the intermediate layer are made.
Accordingly, the invention provides a multi-piece golf ball of at least three layer structure comprising a core, an intermediate layer enclosing the core, and a cover enclosing the intermediate layer. Provided that the core undergoes a deflection A (mm) under an applied load of 100 kg, and a sphere having the core enclosed with the intermediate layer undergoes a deflection B (mm) under an applied load of 100 kg, the ratio of A/B is at least 1.10/1. Provided that the ball has a first coefficient of restitution at a firing velocity of 25 in/s and a second coefficient of restitution at a firing velocity of 50 in/sl the first coefficient minus the second coefficient is less than 0.100.
The cover generally has a plurality of dimples in its surface. In one preferred embodiment, the total of dimple volumes is from 250 mm3 to 450 mm3, provided that the volume of each dimple is determined with respect to an imaginary spherical surface which is given on the assumption that no dimples are formed in the ball surface. Preferably, the core is composed primarily of a rubber composition, the intermediate layer is composed primarily of a thermoplastic resin, and the cover is formed of a thermoplastic resin having a Shore D hardness of up to 62. Also preferably the deflection A of the core under an applied load of 100 kg is 2.5 to 6.5 mm.
The multi-piece golf ball of the invention travels a satisfactory distance even when hit by high-head-speed players, typically professional and low-handicap golfers.
Since the performance of the ball does little depend on the head speed and the coefficient of restitution remains substantially unchanged when hit with clubs of differing numbers, the ball travels a distance exactly corresponding to a particular number of club used.